I recently started making all kinds of bacteria-processed foods by myself (such as yoghurt, but also Sauerteig and so on), and I found myself wondering how much of the weight of these products is actually due to the bacteria themselves in the end. But I couldn't find anything useful on the internet.

So, specifically for yoghurt, how much of its weight is due to the bacteria, i.e. dead and/or living bacterial cells, but not including the products of their metabolism outside the cells?

I would also be happy about numbers on other bacteria- or fungi-processed foods, but I wanted to keep the question simple.

1 Answer
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Interesting question and it will only be possible to do some estimation. The problem is to know the exact bacterial composition as well as the mass of the bacteria. To start, it is necessary to know the number of bacteria in a defined amount of yoghurt. There are scientific publications on this topic, but the numbers in fresh produced yoghurt are in the area of 108 bacteria per gram yoghurt and declines over time. Based on the numbers in reference 1 and 2 I use 2x108 bacteria per gram fresh yoghurt.

The other number we need is the mass of one bacterium. This is complicated, as bacteria are very light due to their size and cannot be easily weighted. There are some numbers available (we imply here that all bacteria weight the same, which is of course not the case), and according to them, one bacterium weighs about 1x10-12 grams.

Mutiplying both numbers gives the amount of bacteria in one gram of yoghurt, around 0.0002 gram or 0.2mg. One typical sized yoghurt of 150 gram therefore contains about 30mg of bacteria. The weight of the bacteria can thus be neglected against the total weight.

$\begingroup$Thanks for the analysis and for pulling out the references! As far as I understand the second reference, the number there seems to be for living bacteria. Do you know any estimates about dead bacteria? Do they just not die as long as they're producing the yoghurt, so that freshly produced yoghurt contains all the bacteria that ever lived in the yoghurt and they start dying afterwards?$\endgroup$
– fifaltraJan 16 '15 at 3:41

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$\begingroup$I think the number is highest in the beginning (when the conditions are optimal) and then the bacteria die. Since they are not removed, the number should still fit.$\endgroup$
– Chris♦Jan 16 '15 at 10:21